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Python3 cheatsheet

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Python sheet to all new comers, who want to get use to the syntax for python3.

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Python3 cheatsheet

  1. 1. Sequence Containers Indexing Base Types Python 3 Cheat Sheet©2012-2015 - Laurent Pointal License Creative Commons Attribution 4 Official Python documentation on http://docs.python.org/py3k 0783 -192int 9.23 -1.7e-60.0float True Falsebool "OnenTwo" 'I'm' str """XtYtZ 1t2t3""" ×10 -6 escaped tab escaped new line Multiline string: Container Types list [1,5,9] ["x",11,8.9] ["mot"] [] tuple (1,5,9) 11,"y",7.4 ("mot",) () dict {1:"one",3:"three",2:"two",3.14:"π"} {"key":"value"} set {} {1,9,3,0} ◾ ordered sequences, fast index access, repeatable values set() ◾ key containers, no a priori order, fast key acces, each key is unique {"key1","key2"} Non modifiable values (immutables) Variables assignment x=1.2+8+sin(y) y,z,r=9.2,-7.6,0 a…zA…Z_ followed by a…zA…Z_0…9 ◽ diacritics allowed but should be avoided ◽ language keywords forbidden ◽ lower/UPPER case discrimination ☝ expression with just comas →tuple dictionary collection integer, float, boolean, string, bytes Identifiers ☺ a toto x7 y_max BigOne ☹ 8y and for x+=3 x-=2 increment ⇔ x=x+3 decrement ⇔ x=x-2 Conversions for lists, tuples, strings, bytes… int("15") → 15 int("3f",16) → 63 can specify integer number base in 2nd parameter int(15.56) → 15 truncate decimal part float("-11.24e8") → -1124000000.0 round(15.56,1)→ 15.6 rounding to 1 decimal (0 decimal → integer number) bool(x) False for null x, empty container x , None x or False x ; True for other x str(x)→ "…" representation string of x for display (cf. formating on the back) chr(64)→'@' ord('@')→64 code ↔ char repr(x)→ "…" literal representation string of x bytes([72,9,64]) → b'Ht@' list("abc") → ['a','b','c'] dict([(3,"three"),(1,"one")]) → {1:'one',3:'three'} set(["one","two"]) → {'one','two'} separator str and sequence of str → assembled str ':'.join(['toto','12','pswd']) → 'toto:12:pswd' str splitted on whitespaces → list of str "words with spaces".split() → ['words','with','spaces'] str splitted on separation str → list of str "1,4,8,2".split(",") → ['1','4','8','2'] sequence of one type → list of another type (via comprehension list) [int(x) for x in ('1','29','-3')] → [1,29,-3] type(expression) lst=[10, 20, 30, 40, 50] lst[1]→20 lst[-2]→40 0 1 2 3 4 -5 -4 -3 -1-2 Individual access to items via lst[index] positive index negative index 0 1 2 3 54 -5 -4 -3 -1-2negative slice positive slice Access to sub-sequences via lst[start slice:end slice:step] len(lst)→5 lst[1:3]→[20,30] lst[::2]→[10,30,50] lst[-3:-1]→[30,40] lst[:3]→[10,20,30]lst[:-1]→[10,20,30,40] lst[3:]→[40,50]lst[1:-1]→[20,30,40] lst[:]→[10,20,30,40,50] Missing slice indication → from start / up to end. On mutable sequences (list), remove with del lst[3:5] and modify with assignment lst[1:4]=[15,25] Conditional Statement if age<=18: state="Kid" elif age>65: state="Retired" else: state="Active" Boolean Logic Statements Blocks parent statement: statement block 1… ⁝ parent statement: statement block2… ⁝ next statement after block 1 indentation ! Comparators: < > <= >= == != ≠=≥≤ a and b a or b not a logical and logical or logical not one or other or both both simulta- -neously if logical condition: statements block statement block executed only if a condition is true Can go with several elifi, elif... and only one final else. Only the block of first true condition is executed. lst[-1]→50 lst[0]→10 ⇒ last one ⇒ first one x=None « undefined » constant value Maths Operators: + - * / // % ** × ÷ integer ÷ ÷ remainder ab from math import sin,pi… sin(pi/4)→0.707… cos(2*pi/3)→-0.4999… sqrt(81)→9.0 √ log(e**2)→2.0 ceil(12.5)→13 floor(12.5)→12 escaped ' ☝ floating numbers… approximated values angles in radians (1+5.3)*2→12.6 abs(-3.2)→3.2 round(3.57,1)→3.6 pow(4,3)→64.0 for variables, functions, modules, classes… names Mémento v2.0.3 str (ordered sequences of chars / bytes) (key/value associations) ☝ pitfall : and and or return value of a or of b (under shortcut evaluation). ⇒ ensure that a and b are booleans. (boolean results) a=b=c=0 assignment to same value multiple assignments a,b=b,a values swap a,*b=seq *a,b=seq unpacking of sequence in item and list bytes bytes b"totoxfe775" hexadecimal octal 0b010 0xF30o642 binary octal hexa "" empty dict(a=3,b=4,k="v") Items count ☝ keys=hashable values (base types, immutables…) True False True and False constants ☝ configure editor to insert 4 spaces in place of an indentation tab. lst[::-1]→[50,40,30,20,10] lst[::-2]→[50,30,10] 1) evaluation of right side expression value 2) assignment in order with left side names = ☝ assignment ⇔ binding of a name with a value ☝ immutables On mutable sequences (list), remove with del lst[3] and modify with assignment lst[4]=25 del x remove name x b"" @ → matrix × python3.5+numpy ☝ index from 0 (here from 0 to 4) frozenset immutable set Priority (…) ☝ usual priorities modules math, statistics, random, decimal, fractions, numpy, etc. (cf. doc) Modules/Names Imports from monmod import nom1,nom2 as fct module truc⇔file truc.py →direct acces to names, renaming with as import monmod →acces via monmod.nom1 … ☝ modules and packages searched in python path (cf sys.path) ? yes no shallow copy of sequence ? yes no and *= /= %= … ☝ with a boolean var x: if x==True: ⇔ if x: if x==False: ⇔ if not x: Exceptions on Errors raise Exception(…) Signaling an error: Errors processing: try: normal procesising block except Exception as e: error processing block normal processing error processing error processing raise raise null ☝ finally block for final processing in all cases.
  2. 2. "modele{} {} {}".format(x,y,r) "{selection:formating!conversion}" ◽ Selection : 2 nom 0.nom 4[key] 0[2] str Displayprint("v=",3,"cm :",x,",",y+4) print options: ◽ sep=" " items separator, default space ◽ end="n" end of print, default new line ◽ file=f print to file, default standard output items to display : literal values, variables, expressions loop on dict/set ⇔ loop on keys sequences use slices to loop on a subset of a sequence Conditional Loop Statementstatements block executed as long as condition is true while condition logique: statements block s = 0 i = 1 while i <= 100: s = s + i**2 i = i + 1 print("sum:",s) initializations before the loop condition with a least one variable value (here i) s= ∑ i=1 i=100 i2 ☝ make condition variable change ! Iterative Loop Statementstatements block executed for each item of a container or iterator for var in sequence: statements block s = "Some text" cnt = 0 for c in s: if c == "e": cnt = cnt + 1 print("found",cnt,"'e'") Go over sequence's values Algo: count number of e in the string. Go over sequence's index ◽ modify item at index ◽ access items around index (before / after) lst = [11,18,9,12,23,4,17] lost = [] for idx in range(len(lst)): val = lst[idx] if val> 15: lost.append(val) lst[idx] = 15 print("modif:",lst,"-lost:",lost) Algo: limit values greater than 15, memorizing of lost values. ☝bewareofinfiniteloops! initializations before the loop loop variable, assignment managed by for statement Operations on Strings values to formatformating directives Integers Sequences Files s = input("Instructions:") ☝ input always returns a string, convert it to required type (cf. boxed Conversions on the other side). range(5)→ 0 1 2 3 4 range(2,12,3)→ 2 5 8 11 range(3,8)→ 3 4 5 6 7 range(20,5,-5)→ 20 15 10 range(len(seq))→ sequence of index of values in seq ☝ range provides an immutable sequence of int constructed as needed range([start,] end [,step]) f = open("fil.txt","w",encoding="utf8") storing data on disk, and reading it back opening mode ◽ 'r' read ◽ 'w' write ◽ 'a' append ◽ …'+' 'x' 'b' 't' encoding of chars for text files: utf8 ascii latin1 … name of file on disk (+path…) file variable for operations f.write("coucou") writing ☝ if text file → read / write only strings, convert from/to required type reading s = f.read(4) with open(…) as f: for line in f : # processing ofline cf. modules os, os.path and pathlib if char count not specified, read whole file s = f.readline() read next line f.close() ☝ dont forget to close the file after use ! Very common: opening with a guarded block (automatic closing) and reading loop on lines of a text file: Function Definition def fct(x,y,z): """documentation""" # statements block, res computation, etc. return res function name (identifier) result value of the call, if no computed result to return: return None ☝ parameters and all variables of this block exist only in the block and during the function call (think of a “black box”) named parameters Function Callr = fct(3,i+2,2*i) Generic Operations on Containers empty string if end of file len(c)→ items count min(c) max(c) sum(c) sorted(c)→ list sorted copy val in c → boolean, membership operator in (absence not in) enumerate(c)→ iterator on (index, value) zip(c1,c2…)→ iterator on tuples containing ci items at same index all(c)→ True if all c items evaluated to true, else False any(c)→ True if at least one item of c evaluated true, else False ☝ modify original list lst.append(val) add item at end lst.extend(seq) add sequence of items at end lst.insert(idx,val) insert item at index lst.remove(val) remove first item with value val lst.pop([idx])→value remove & return item at index idx (default last) lst.sort() lst.reverse() sort / reverse liste in place "{:+2.3f}".format(45.72793) →'+45.728' "{1:>10s}".format(8,"toto") →' toto' "{x!r}".format(x="I'm") →'"I'm"' ☝ start default 0, fin not included in sequence, pas signed default 1 ◽ Conversion : s (readable texte) or r (literal representation) < > ^ = 0 at start for filling with 0 integer: b binary, c char, d decimal (default), o octal, x or X hexa… float: e or E exponential, f or F fixed point, g or G appropriate (default),  string: s … % percent ◽ Formating : fill char alignment sign mini width.precision~maxwidth type + - space Operations on Dictionaries Operations on Sets Operators: | → union (vertical bar char) & → intersection - ^ → différence/symetric diff. < <= > >= → inclusion relations Operators also exist as methods. d.update(d2) update/add associations Note: For dictionaries and sets, these operations use keys. Specific to ordered sequences containers (lists, tuples, strings, bytes…) reversed(c)→ inversed iterator c*5→ duplicate c+c2→ concatenate c.index(val)→ position c.count(val)→ events count Opérations on Lists d[key]=value d[key]→ value d.keys() d.values() d.items() d.clear() del d[key] →iterable views on keys/values/associations Examples d.pop(key[,default])→ value d.popitem()→ (key,value) d.get(key[,default])→ value d.setdefault(key[,default])→value s.update(s2) s.copy() s.add(key) s.remove(key) s.discard(key) s.clear() s.pop() Loop Control Go simultaneously on sequence's index and values: for idx,val in enumerate(lst): ☝goodhabit :don'tmodifyloopvariable Advanced: def fct(x,y,z,*args,a=3,b=5,**kwargs): *args variable positional arguments (→tuple), default values, **kwargs variable named arguments (→dict) one argument per parameter storage/use of returned value Algo: f.flush() write cache f.tell()→position reading/wriding progress sequentially in the file, modifiable with: f.seek(position[,origin]) f.truncate([taille]) resize Formating Advanced: *sequence **dict s.startswith(prefix[,start[,end]]) s.endswith(suffix[,start[,end]]) s.strip([chars]) s.count(sub[,start[,end]]) s.partition(sep)→ (before,sep,after) s.index(sub[,start[,end]]) s.find(sub[,start[,end]]) s.is…() tests on chars categories (ex. s.isalpha()) s.upper() s.lower() s.title() s.swapcase() s.casefold() s.capitalize() s.center([width,fill]) s.ljust([width,fill]) s.rjust([width,fill]) s.zfill([width]) s.encode(encoding) s.split([sep]) s.join(seq) ? yes no next finish … Input import copy copy.copy(c)→ shallow copy of container copy.deepcopy(c)→ deep copy of container ☝ this is the use of function name with parenthesis which does the call fct() fct fct ☝ text mode t by default (read/write str), possible binary modeb (read/write bytes) break immediate exit continue next iteration ☝ else block for normal loop exit.

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